Reported as polymer electrolyte materials are polymers categorized in so-called anion-exchange resins, for instance, polystyrene sulfonic acid, polyvinyl sulfonic acid, perfluorosulfonic acid polymers, perfluorocarboxylic acid polymers [Polymer Preprints, Japan Vol. 42, No. 7, pp. 2490–2492 (1993), Polymer Preprints, Japan Vol. 43, No. 3, pp. 735–736 (1994), Polymer Preprints, Japan Vol. 42, No. 3, pp. 730 (1993)], etc.
Particularly because polymer materials having sulfonic acid groups in side chains tend to be strongly bonded to particular ions and selectively permit cations or anions to permeate therethrough, they are formed into particles, fibers or membranes for use in various applications such as electrodialysis membranes, diffusion dialysis membranes, battery separator membranes, etc. Among others, electrolyte membranes of fluorine-containing polymers having perfluoro-skeletons with sulfonic acid groups bonded to side chains, which are available from DuPont under the trademark of Nafion, are excellent in heat resistance and chemical resistance, so that they are put into practical use as electrolyte membranes capable of withstanding use under severe conditions. However, the above fluorine-containing electrolyte membranes are disadvantageous in difficulty in production and thus extremely expensive.
Also reported is a polymer electrolyte made of a heat-resistant resin having a hydrocarbon skeleton such as polybenzimidazole, etc., into which an alkyl sulfonic acid group or an alkyl phosphoric acid group is introduced (JP 9-87570 A, JP 9-110982 A). This polymer electrolyte exhibits high electric conductivity (10−4–10−2 Scm−1) and excellent heat resistance (weight decrease initiation temperature: 250° C. or higher) at a high temperature of 100° C. in a moist state. However, it does not exhibit any electric conductivity in a dry state, and requires the use of a toxic solvent such as dimethylacetamide, etc. in the production of a cast film. Also, because the heat-resistant resin such as polybenzimidazole, etc. is extremely expensive, it is disadvantageous as common materials for automobile fuel cells, etc. from the aspect of cost performance. There is also a report that because resins having polybenzimidazole skeletons and those having hydrocarbon skeletons are generally poor in acid deterioration resistance, they are disadvantageous in durability (JP 2000-11755 A). It is thus desired to provide polymer electrolytes having higher electric conductivity.